Methane from natural gas is an important raw material. North America alone has an estimated 1160 trillion cubic feet (tcf) of proven natural gas reserves. If captured and converted, the gas would, after conversion losses, enable 250 billion barrels of synthetics, which can be used from clean-burning diesel to jet fuel. Yet current high-temperature hydrocarbon conversion technologies are expensive and inefficient. Therefore, there is a need for low-temperature gas to liquid (LT-GTL) technologies. Another important challenge is that of the estimated 6600 tcf worldwide natural gas proven reserves, some 30%-60% are classified as stranded gas, i.e., gas that has been discovered but remain unusable for either physical or economic reasons.
The need to capture and utilize natural gas resources efficiently as an alternative chemical feedstock is becoming more urgent due to diminishing proven reserves and increasing consumption of crude oil. These trends are reflected in the expectation that unconventional gas production will grow significantly. Although establishing skid-mounted LT direct hydrocarbon conversion technology to capture existing reserves fully and efficiently has been a top priority, it has been met with the two challenges described above.
Returning to examine these challenges in some detail in reverse order, it is noted first there are geographic barriers to exploiting stranded gas economically. In geographically remote locations, pipeline infrastructure is lacking. Also, although there is gas associated with crude oil production operations, the quantities are often sub-economic to permit transport to market. Additionally, from offshore deepwater oil production, gas is produced that is difficult to capture. Second, the current dominant GTL technology is energy-intensive, requiring operations at temperatures generally greater than 700 C; capital-intensive, as expensive metal oxidants are required; and technically lacking, giving low conversion and carbon selectivity rates, the latter being 60% maximum.
These barriers have negative economic and environmental consequences. The former include an inability to tap vast reserves of stranded gas, high capital costs that amount to greater than 40% of production costs, and prevention of small- and middle-scale gas exploration. While among pollution concerns are the fact that the majority of explored gas is flared, creating greenhouse effects, and that current processes have low carbon efficiency. Offshore associated gas poses a special set of challenges, where skid-mounted GTL technology does not exist, production rates fluctuate, production volume is sub-economic, and high capital expenditures are required for gas capture and export. As a result, 150 billion cubic meter of gas is flared per year, equivalent to about 25% of the US's or 30% of the EU's gas consumption, per year.
Focusing on the technical aspects of the current GTL technology, one notes that methanol production via synthesis gas generation is the dominant technology in the natural gas market. As the major species in natural gas, methane's carbon-hydrogen bond has a high bond energy of 439 kJ/mol, and as a result is extremely insert to reactivity. In the dominant current approach, methane is oxidized in an early step at 700 C-1000 C to produce a gas mixture of carbon monoxide and hydrogen, known as synthesis gas or syngas, which is further catalytically converted to methanol. Methanol is then used as a major feedstock to make formaldehyde, acetic acid, methyl chloride, olefins, gasoline additives, and other chemicals and products. Methanol can also be used directly as a fuel for vehicles. However, the dominant current approach calls for high temperatures and is therefore expensive to operate. It also has many steps in the process and requires high capital expenditure. As well, there is no skid-mounted plant that is currently economically applicable or feasible.
Attempts have been made to improve the natural gas GTL process in the past decades. One line of research has been to use methane to produce monohalogenated methane, which can then be further processed. GRT Incorporated, a US company located in Santa Barbara, for example, has developed a novel, fundamentally simpler, and more direct approach to GTL processes, whereby rather than first generating synthesis gas from methane, methane is activated by bromine (Br2), reaction being captured by the following Equation (1):CH4+Br2-->CH3Br+CH2Br2+CHBr3  (1)
By eliminating the need to form synthesis gas, the GRT process is more economically applicable to biomethane and to natural gas from small- and medium-size stranded gas fields. Dow Chemical also developed a technology to convert methane to methyl chloride, and again di- and tri-chloromethanes are major products in the product mixture. Their technology may be shown as follows by Equation (2):CH4+Cl2-->CH3Cl+CH2Cl2+CHCl3+HCl  (2)
The main difficulty of both improved approaches is carbon selectivity. That is, while a monohalogenated product would be most desirable, reactions in both cases do not stop at the monohalogenated methane product, but instead proceed to allow polyhalogenated products as major species in the product mixture. Because additional separation steps are economically unattractive, there is a need to develop methods of producing methyl halide without unwanted byproducts.
Other references in the art teach processes to halogenate methane. For example, U.S. patent application Ser. No. 11/912,376 describes an oxidative halogenation process for preparing a halogenated C1 product by contacting methane or a C1 halogenated hydrocarbon with a source of oxygen, a source of halogen, and a catalyst, at specific molar ratios of reactant hydrocarbon to oxygen and/or halogen. (The same reference, incidentally, points to a long sought need in the art for a solution to convert natural gas to useful chemical feedstocks, echoing the description provided above.) U.S. patent application Ser. No. 13/123,908 describes another process to oxidatively halogenate methane, by placing a feedstream that comprises methane, a source of halogen, a source of oxygen, and a source of diluents gas in contact with a first and then a second catalyst.
The '376 application, however, describes a process that requires a high temperature for the reaction to occur, listing a general range that is greater than about 375 C and less than about 700, and is relatively cumbersome to use. The '908 application also describes an involved process that requires operation at higher temperatures (at a range of 200 C to 600 C) in at least one step in the process, does not offer great selectivity for the monohalogenated species, and produces several impurities, including carbon monoxide. The '908 application additionally produces water and hydrogen chloride, which as taught by the '376 application will allow formation of an azeotrope from which it is difficult and expensive to separate dry hydrogen chloride for recycling purposes.
Moreover, neither process specifies the appearance of methanesulfonyl halogen or methanesulfonyl chloride in the reaction process or their use as reaction intermediates.
Methanesulfonyl chloride (MSC), a liquid at room temperature (boiling point=161 C), is a compound that methane can react with to form according to Equation (3):CH4+SO2Cl2--(Urea-H2O2,RhCl3,60 C,12 h,H2SO4 solvent)-->CH3SO2Cl+HCl  (3)
Although a versatile reagent that has several uses, among them as a mesyl group introduction species, a synthetic intermediate for photographic chemicals and agrochemicals, a stabilizer or catalyst, and a precursor to methanesulfonic acid (Mukhopadhyay et al., Chemical Communications, pp. 472-473 (2004), incorporated herein by reference), MSC's commercial market remains quite limited compared to such basic chemicals as for example methanol, light olefins, and dimethyl ether.
Further, because MSC is highly toxic, moisture sensitive, corrosive, and a lachrymator, it has not been thought of by previous references in the art as a compound that can serve as a key intermediate for methane conversion into useful chemicals, although it can be formed from methane. For example, U.S. Pat. No. 4,997,535 teaches a process to manufacture MSC from a mixture of methane, SO2 gas, and Cl2 gas under irradiation of light with wavelengths of 200˜600 nm, and U.S. Pat. No. 6,045,664 describes a method to produce MSC by photo-chemical reaction of CH4 with Cl2 and SO2, during which process a small amount of chlorinated methane in the form of a byproduct (less than 1%) was observed. Neither of these references describes useful thermal decomposition products of MSC, and in particular does not mention methyl chloride or chloromethane.
Additional references for the thermal decomposition products of methanesulfonyl chloride, as provided by the MSDS from three of the largest chemical providers are listed in completion as follows. (1) Acros Organics MSDS: hydrogen chloride, carbon monoxide, oxides of sulfur (SOx), carbon dioxide; (2) Arkema Inc. (which supplies Sigma-Aldrich) MSDS: methansulfonic acid (CH3SO3H), sulfur oxides (SOx), and carbon oxides (COx); and (3) Fisher Scientific MSDS: hydrogen chloride, carbon monoxide, oxides of sulfur, carbon dioxide. It is noted that methyl chloride does not appear among the listed thermal decomposition products of methanesulfonyl chloride.
Methyl chloride, a monohalogenated methane species that has been sought after in the art as a relatively pure reaction product, however, is an important commodity chemical. According to the 1998 “Toxicological Profile for Chloromethane” published by the Agency for Toxic Substances and Disease Registry of the U.S. Department of Health and Human Services, and its 2009 addendum, there are two common large-scale industrial methods to produce chloromethane or methyl chloride: methanol-HCl and methane chlorination. In the methane chlorination process, after HCl removal, a fractional distillation step is necessary to separate the four chlorinated methanes and isolate the mono-chlorinated product.
The same sources gave the US production amount in 1995 to be around 920 million ponds (417.3 million kg). As of 1998, there were at least 96 facilities in the U.S. that produced chloromethane, of which seven had production capacities in excess of 50 million ponds per year. At three of these, all the chloromethane generated were used on-site in silicone production, while at the other four, a large percentage of the output were also used on-site as feedstocks in the manufacture of other chemicals and products. Overall, chloromethane as of 1995 was used mainly (72%) in the production of silicones. Chloromethane has also been used in the production of agricultural chemicals (8%), methyl cellulose (6%), quaternary amines (5%), butyl rubber (3%), and for miscellaneous uses including tetramethyl lead (2%).
Therefore one sees that regarding methyl chloride, (1) one of the two processes for its production in large-scale current use employs methane as a reactant, but requires distillation to separate out the mono-chlorinated species; and (2) its feedstock use is of significance.